Radio frequency (RF) switches are important building blocks in many wired and wireless communication systems. RF switches are found in many different communication devices such as cellular telephones, wireless pagers, wireless infrastructure equipment, satellite communications equipment, and cable television equipment. As is well known, the performance of RF switches may be characterized by one of any number operating performance parameters including insertion loss and switch isolation. Performance parameters are often tightly coupled, and any one parameter can be emphasized in the design of RF switch components at the expense of others. Other characteristics that are important in RF switch design include ease and degree (or level) of integration of the RF switch, complexity, yield, return loss and, of course, cost of manufacture.
FIG. 5 shows a pseudomorphic High Electron Mobility Transistor (pHEMT) RF switch 500 according to the prior art. The RF switch 500 includes an RF common (RFC) input node 501 and two RF output nodes 502, 503. Coupling/DC blocking capacitors are also shown at each of the RFC input node 501 and the RF output nodes 502, 503 but are ignored for purposes of the instant description. Those skilled in the art will appreciate that such capacitors impede the passage of DC current, yet do not appreciably impact an AC signal.
As further shown, several transistors M51, M52, M53, and M54 are arranged to effect RF communication between the RFC input node 501 and the RF output node 502, or between the RFC input node 501 and the RF output node 503. Specifically, the transistor M51 is arranged between the RFC input node 501 and the RF output node 502, the transistor M52 is arranged between the RF output node 502 and ground, the transistor M53 is arranged between the RFC input node 501 and the RF output node 503, and the transistor M54 is arranged between the RF output node 503 and ground. Each of the transistors M51-M54 includes by-pass resistors (which are not labeled with reference numerals) connected between respective drain and source terminals.
Two control signals VC1 and VC2 applied, respectively, to the gates of the transistors M51 and M53 control which path (the RFC input node 501 to the RF output node 502 or the RFC input node 501 to the RF output node 503) will be taken by an RF AC signal input at the RFC input node 501. In the configuration shown, the control signal VC1 is 3.3V, which turns the transistor M51 on. The control signal VC2 is 0V, which turns the transistor M53 off. In this configuration, the RF path is configured to be the RFC input node 501 to the RF output node 502. The transistors M52 and M54 operate to enable either an isolation branch or a shunt branch depending on which path (the RFC input node 501 to the RF output node 502 or the RFC input node 501 to the RF output node 503) is selected. That is, when the control signal VC1 is high (3.3V), a control signal VC1B applied to the gate of the transistor M52 is controlled to be low, e.g., 0V. With the control signal VC1B low, the transistor M52 is off thereby isolating the path between the RFC input node 501 and the RF output node 502. Meanwhile, a control signal VC2B is set high or equal to the control signal VC1 thus turning the transistor M54 on and creating an AC signal shunt path between the RF output node 503 and ground to ensure that no signal (or very little) is present at the RF output node 503 when the RF output node 503 is not selected to output the AC signal received at the RFC input node 501. As shown in FIG. 5, the several control signals VC1, VC1B, VC2, and VC2B are applied via respective resistors (which are also not labeled with reference numerals).
For high power operation for the RF switch 500 shown in FIG. 5, a voltage of a node 525 must be high enough to not only make the transistor M51 forward biased with a positive Vgs over drive (the control signal VC1−“the voltage at the node 525) to reduce turn-on insertion loss, but also to maintain sufficient reverse bias of the transistor M52 (0− “the voltage at the node 525”) to avoid turn-on during high power voltage swings.
For a depletion pHEMT device, threshold voltage (Vth) is about −1V. Due to the relatively large leakage current in a pHEMT device, two back-to-back diodes 520, 521 form a Kirchoff Voltage Law (KVL) node. Specifically, with the control signal VC1=3.3 voltage, the drop across the diode 520 is approximately 0.7V, which causes a voltage of 2.6V to be present at the node 525. With a voltage of 2.6V at the node 525, Vgs for the transistor M52 (which is reversed biased) is 0V−2.6V, or −2.6V. The point here is that as a result of the leakage current, the node 525 is set at 2.6V which is suitable for handling high power RF switch operations, and as a result, no auxiliary biasing is needed to support a high power RF switch implemented with pHEMT devices.
Unlike pHEMT device-based RF switches, silicon-based RF switches permit much less leakage current. As such, silicon-based RF switches operating in high power scenarios require special biasing circuitry and voltages to operate properly. There is accordingly a need to provide cost effective ways of providing such biasing circuitry and voltages in silicon-based RF switching devices.